1. Field of the Invention
The present invention relates to an inverter control apparatus, and in particular to an inverter control apparatus using a two-phase modulation method whereby the voltage duty ratio of a certain phase of an inverter is fixed to 0% or 100% at a predetermined phase.
2. Description of the Related Art
For PWM control apparatuses of three-phase inverters, a two-phase modulation method capable of decreasing the electromagnetic noise and chopping loss and increasing the power conversion efficiency has been conventionally advocated. According to this two-phase modulation method, the voltage level of a certain phase out of three phases is fixed to a high level or a 0 level and modulation is performed by using two remaining phases. A typical two-phase modulation method is disclosed in U.S. Pat. No. 4,641,075 by Asano et al.
In the above described known two-phase modulation method, switching devices of output stage of each phase of a three-phase inverter are controlled to be opened and closed according to a current difference between a current instruction value of each phase and a detected current value of each phase, thus controlling the duty ratio of three-phase voltage applied to the terminals of each phase of a three-phase load by the output stage of each phase of the three-phase inverter. In this case, generation of undesired voltage vectors is reduced by fixing current instruction values of respective phases to their maximum value during respectively different predetermined phase intervals.
Furthermore, it is assumed in the above described known two-phase modulation method that the power factor of the load is constant and the ideal phase voltage associated with a current instruction value can be easily estimated on the basis of the output current instruction value. Supposing an interval during which the ideal phase voltage estimated from a known power factor .theta.v of the load regarded as constant assumes its peak, the amplitude of the current instruction value of its phase is fixed to its maximum value during the interval and the current instruction value of each phase is provided with a sine waveform during remaining intervals. During an interval that the ideal phase voltage assumes its peak, therefore, the voltage of the phase associated with a fixed current instruction value is fixed to the maximum value. As a result, the applied voltage of each phase is somewhat distorted, but has a waveform substantially equivalent to that of a three-phase AC voltage. So long as detected current values of the two remaining phases exactly follow the current instruction values, the detected current value of the phase associated with the fixed current instruction value should have a sine waveform.
Thus, in the two-phase modulation method disclosed in U.S. Pat. No. 4,641,075, the ideal phase voltage for a given current instruction value is estimated on a certain supposition and the current of a certain phase is fixed to the maximum value during a certain interval on the basis of this supposition. The two-phase modulation method has the following problems.
A first problem will now be described. In conventional motors, the phase angle (also referred to as phase difference) of a load current flowing through the load and voltage applied to the load varies according to the magnitude of the load current and the frequency of voltage applied to the load. (The internal impedance of the motor varies according to the magnitude of the load current. The internal impedance of the motor varies according to the frequency.) In the case where the phase difference is supposed to be constant as described above, therefore, the phase voltage of that load cannot be fixed to the maximum value in the interval during which the voltage applied to the load assumes its peak. As a result, distortion of the voltage waveform applied to the load (motor) increases, resulting in a lowered efficiency, heat generation, and increased noise. That is to say, a favorable result is obtained when phase difference is supposed to be constant. Under some power factors of the motor, however, the voltage of the phase is fixed to the maximum value a timing where the voltage duty ratio should not assume a high level.
A second problem will now be described. The above described first problem has been described as to the case where the control system is handled as an AC circuit for processing sine wave signals. As a matter of fact, however, the waveform to be processed becomes a non-sinusoidal waveform. To be precise, therefore, it becomes necessary to consider the response delay of the transfer function in the impedance system, including reactance of the motor. That is to say, delay of the detected current based on transfer phase delay of the control system as compared with the current instruction value further increases phase deviation of the phase of the actual phase voltage shifted by an actual phase difference as compared with the phase of the detected current from the phase of the phase voltage shifted by a predetermined (prospective) phase difference as compared with a current instruction value of a certain phase.
The aforementioned problems will further be described. In the conventional two-phase modulation method, duty ratio control of an inverter is effected on the basis of deviation of the current instruction value, which typically has a sine waveform, from the detected current (also referred to as actual current value). Thereby the voltage applied to the motor is controlled.
In the impedance system including the reactance of the motor, the detected current (actual current value) is delayed by a predetermined phase angle from the applied voltage having a sine waveform as described above. In the impulse response, a predetermined response delay is caused. Therefore, the impedance system has a characteristic where the response performance of the above described deviation, i.e., response performance of the detected current to a change of applied voltage is worsened considerably.
In the conventional technique, therefore, the phase of the above described deviation of the detected current from the current instruction value (or voltage applied to the load proportionate thereto) is shifted so as to put voltage of a certain phase ahead of the current instruction value of that phase by a predetermined fixed phase difference. However, the phase of deviation (or applied voltage proportionate thereto) is not shifted with respect to the above described problem of occurrence of deviation from the phase difference and the problem of the above described impulse response delay characteristic of the actual current value. Therefore, there occurs a problem that this response delay cannot be compensated in phase. As a result of them, distortion of the applied voltage waveform is increased and the actual current value is also distorted thereby. This resulted in a problem of occurrence of noise and a lowered efficiency.
By the way, there is disclosed by Kamiyama in U.S. Pat. No. 4,847,743 a method for reducing the switching loss of the two-phase modulation method and minimizing the distortion factor of the output voltage. According to Kamiyama's method, a two-phase modulation circuit and a three-phase modulation circuit are provided and used in combination.